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What is interface in JavaInterface in java is core part of Java programming language and one of the way to achieve abstraction in Java along with abstract class. Even though interface is fundamental object oriented concept ; Many Java programmers thinks Interface in Java as advanced concept and refrain using interfacefrom early in programming career. At very basic level interface in java is a keyword but same time it is an object oriented term to define contracts and abstraction , This contract is followed by any implementation of Interface in Java. Since multiple inheritance is not allowed in Java, interfaceis only way to implement multiple inheritance at Type level. In this Java tutorial we will see What is an interface in Java, How to use interface in Java and where to use interface in Java and some important points related to Java interface. What is an interface in Java is also a common core Java question which people asked on various programming exams and interviews.

Key Points about Interface in Java

In last section we saw What is an interface in Java and learned that interface provides abstraction in Java and its only way to achieve multiple inheritance at type level in Java. In this section we will see some important properties of interface in Java.

1. Interface in java is declared using keyword interface and it represent a Type like any Class in Java. a reference variable of type interface can point to any implementation of that interface in Java. Its also a good Object oriented design principle to "program for interfaces than implementation" because when you use interface to declare reference variable, method return type or method argument you are flexible enough to accept any future implementation of that interface which could be much better and high performance alternative of current implementation. similarly calling any method on interface doesn't tie you with any particular implementation and you can leverage benefit of better or improved implementation over time. This maintenance aspect of interface is also sought in various software design interview questions in Java.

2) All variables declared inside interface is implicitly public final variable or constants. which brings a useful case of using Interface for declaring Constants. We have used both Class and interface for storing application wide constants and advantage of using Interface was that you can implement interface and can directly access constants without referring them with class name which was the case earlier when Class is used for storing Constants. Though after introduction of static imports in Java 5 this approach doesn't offer any benefit over Class approach.

3) All methods declared inside Java Interfaces are implicitly public and abstract, even if you don't use public or abstract keyword. you can not define any concrete method in interface. That's why interface is used to define contracts in terms of variables and methods and you can rely on its implementation for performing job.

4) In Java its legal for an interface to extend multiple interface. for example following code will run without any compilation error:

interface Session extends Serializable, Clonnable{ }

Here Session interface in Java is also a Serializable and Clonnable. This is not true for Class in Java and one Class can only extend at most another Class. In Java one Class can implement multiple interfaces. They are required to provide implementation of all methods declared inside interface or they can declare themselves as abstract class.

Example of interface in Java

Java standard library itself has many inbuilt interfaces like Serializable, Clonnable, Runnableor Callable interface in Java. Declaring interface is easy but making it correct in first attempt is hard but if you are in business of designing API then you need to get it right in first attempt because its not possible to modify interface once it released without breaking all its implementation. here is an example of declaring interface in Java :

In above example of interface in Java, SessionIDCreator is an interface while SerialSessionIDCreator is a implementation of interface. @Override annotation can be used on interface method from Java 6 onwards, so always try to use it. Its one of those coding practice which should be in your code review checklist.

When to use interface in Java

Interface is best choice for Type declaration or defining contract between multiple parties. If multiple programmer are working in different module of project they still use each others API by defining interface and not waiting for actual implementation to be ready. This brings us lot of flexibility and speed in terms of coding and development. Use of Interface also ensures best practices like "programming for interfaces than implementation" and results in more flexible and maintainable code. Though interface in Java is not the only one who provides higher level abstraction, you can also use abstract class but choosing between Interface in Java and abstract class is a skill. Difference between Interface in Java and abstract class in java is also a very popular java interview question.

That's it for now on What is Interfacein Java, specifics of Java interface and How and when to use Interface in Java. Interface is key to write flexible and maintainable code. If you are not yet using interface in your code than start thinking in terms of interfaces and use it as much possible. You will learn more about interfaces when you start using design patterns. many design patterns like decorator pattern, Factory method pattern or Observer design pattern makes very good use of Java interfaces.

21 comments
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interface is really beautiful concept introduced by Object oriented programming language and Java seems use it perfectly. there are several benefits of using Interface while coding like:

1) Interface offers protection with change in requirement or future enhancement.2) By using interface you can leverage more powerful implementation as highlighted in this article.3) Code written using interface looks clean and extensible.4) Interface can speed up development process and facilitate communication between two different module even if they are not complete.

Now question is how to master art of interface ? any good book which teaches how to design systems in terms of interface ?

@Anonymous and @Prasanna, I think output should be "toString", as show() method is not called any where. As Prasanna pointed out this Child object so toString() from Child class will get called.

Now your second question "Does interface in Java extends Object class", which is indeed a good question.Simple answer is No. interface in Java doesn't extend java.lang.Object class.

You could probably ask, how come p.toString() compiled? because as per Java language specification, interface implicitly declared methods corresponding to java.lang.Object with same signature, until, It is getting those methods from super interface or they are already explicitly declared on interface.

OOPS concepts in briefObject-An object is a software bundle of related state(Color,Height etc) and behavior(Selection).An object stores its state in fields (variables) and exposes its behavior through methods (functions).Ex-Vehicle.

Class-Class is ma collection of objects.

Inheritance-Different kinds of objects often have a certain amount in common with each other. Cars,Bikes etc for example, all share the characteristics of vehicles (speed, gear).class Student{ int rollno; String name;

The syntax for creating a subclass is simple. At the beginning of your class declaration, use the extends keyword, followed by the name of the class to inherit from:

class car extends vehicle {

} Interface-interface is a group of related methods without definition i.e code in them. Ex- Buttons on a Remote act as Interface betweenTV and the viewer. A vehicles behavior, if specified as an interface, might appear as follows:interface Vehicle { void changeGear(int newValue); void speedUp(int increment);}To implement this interface, the name of your class would change (to a particular brand of vehicle, for example, such as car), and you'd use the implements keyword in the class declaration:class car implements vehicle {

}Package-A package is a namespace that organizes a set of related classes and interfaces. Conceptually you can think of packages as being similar to different folders on your computer.

I will add another frequently asked question related to interface in javaWhy you use interface in java if you can not define any method there?Well in reality interface is key for flexible and reusable java program. Suppose an Employee objects needs to calculate its salary and There are different kinds of employees e.g. Hourly, Daily, Monthly. Now how do you write code for that

1. Does your employee needs to know all these salary calculation method? Or every employee needs to know only know its type. Interface save you here.

Inheritance is good for code reuse, but it can cost flexibility, if not used correctly. It's very important for an Object Oriented designer to understand inheritance in Java quite well. Following are some of the strong and week points of Inheritance, I have learned in hard way, might help your readers :

1)Inheritance is great if all sub classes have same behavior, but it's burden if some functionality defined in super-class is applicable to all but and some functionality to only few.

2) Inheritance is inflexible, you can not provide new functionality at runtime. Your object functionality is defined at compile time by methods defined in that class, there is no way to configure object at runtime, that's where composition outscore Inheritance.

3) Inheritance is great for defining types, that too, you should use interface for declaring type. By implementing interface, an object becomes part of a family and can stand for them.

@Anonymous, I suggest you to read Effective Java, Joshua Bloach has suggested some good use of interface in Java, one of them is use interface for declaring type, advantage? One object can represent multiple type because it can implement multiple interface. Another advantage is that using interface will lead you to program for interface than implementation, which is key to create flexible object oriented design in Java.

Though I agree that using interface is great in terms of flexibility, but it also has few disadvantages. For example by coding Interface, you are restricted to methods declared on interface. One example, which I face, where I had to change the type from Map to ConcurrentHashMap, was putIfAbsent(), this method is only defined in ConcurrentHashMap, and not in java.util.Map. If a method accepts java.util.Map and you provide CHM, you can't use putIfAbsent() method. Though, with default method coming in Java 8, Map class is extended to provide few more key methods e.g. replace() and putIfAbsent() etc.

One more use of interface which i think you should include here. Although an interface cannot be instantiated but since it is a type, Java allows to write a method with a parameter of an interface type. That method parameter will accept any class that implements this interface.

In JDK 8 interfaces are allowed to have a default implementation where as earlier versions of JDK does not allow. This is a major update to JDK after a long time. I think it's better to update this article now to keen in sync with latest changes.

Note that from Java 8 interface can have default and static methods. Which means a default implementation for a method can be given. Also static methods can be written which can be accessed using Interface_name.method_name.